To trace the expansion and differentiation of oval cells in rat liver, both pulse-chase labeling experi-ments and the expression of two new growth factor/receptor systems that promote survival and either promote or inhibit differentiation of primitive cells were studied in the acetylaminofluorene/partial hepatectomy (AAF/PH) model. The transfer of tritiated thymidine (3-HTdR) from oval cells to transitional cells and small basophilic hepatocytes, could be detected by autoradiography, but rapid decrease of the immunohistochemical signal in bromodeoxyuridine (BUdR)-labeled cells made it undetectable in the transitional cells and basophilic hepatocytes. Inhibition of growth and function of hepatocytes, as reflected in decrease of serum albumin and increase of alanine aminotransferase (ALT), is required for the prominent differentiation of oval cells into hepatocytes in the AAF/PH model. Stem cell factor (SCF) and its receptor c-kit are present both in oval cells and in the rat liver epithelial (RLE) cell line. In vitro SCF neither promoted the proliferation of RLE cells nor affected their morphology. RLE cells transfected with the mutant c-kit showed a two- to threefold higher rate for apoptosis when grown with delipidated serum or treated with transforming growth factor beta type 1 (TGFbeta-1), as compared to the cells with control plasmid only. Therefore SCF/c-kit may be involved in the survival of liver stem cells. Leukemia inhibitory factor (LIF) was not expressed constitutively but was induced in the nonparenchymal cells from AAF/PH animals and from lipopolysaccharide-treated animals. LIFR transcripts exist constitutively in the hepatocytes and are induced mainly in the nonparenchymal cells of AAF/PH animals. In the Ito cell culture LIF was induced by lipopolysaccharide. This suggests that Ito cells are the major source of LIF in the nonparenchymal cell fraction. Involvement of the TGF-beta family in stem cell biology was studied during pre- and postnatal liver development and during hepatic differentiation in adult liver. Transcripts for TGFbeta-1 and -3 were strongly expressed in the embryonic livers and reached a normal level at 5 weeks after birth. When oval cell proliferation was most prominent, both TGFbeta-1 and -3 reached the peak of their expresssion. A slight increase in TGFbeta-2 expression was also observed during fetal development and hepatic differentiation in adult liver. TGFbeta receptors showed a similar pattern of expression as did TGFbeta peptides. Therefore, the TGFbeta/TGFbeta receptor systems do not inhibit the proliferative activity in these livers, but may promote the differentiation of primitive hepatic cells.